For example, in a case where cardiac surgery of a patient is performed, extracorporeal blood circulation is performed in which a pump is driven by using a motor of an extracorporeal circulator so that blood is removed from the vein (e.g., the vena cava) of the patient, exchange of oxygen gas in the blood is performed through an artificial lung, and then the blood is returned to the artery (e.g., the aorta) of the patient again.
In the extracorporeal circulator, it is important that a lower limit rotation speed of the pump motor is maintained during the time that the patient receives extracorporeal blood flow from the circulator. The lower limit rotation speed of the pump motor denotes a minimum rotation speed for preventing backflow of blood inside a circulation circuit 1R (FIG. 1) when the pump motor is operated so as to cause the blood to return from the artificial lung (artificial heart and lung) to the inside of the body of a patient. If the rotation speed of the motor drops to the lower limit rotation speed or lower, backflow of blood may occur in the circulation circuit and may affect the body of the patient. Thus, it is important that the pump speed that is manually commanded via a control knob on a control unit by a manipulator remains at or above the lower limit rotation speed of the motor. On the other hand, the manipulator needs to have the ability to command a pump speed below the lower limit rotation speed during other stages of the cardiac surgery (such as coast down in order to disconnect the patient from the extracorporeal circulator.
Japanese publication JP-A-5-48439 discloses a counter device which is attached to various types of industrial instruments and performs various types of control by counting the number of times of operations of the instrument. The counter device has a case, a counting setting unit which is provided in the case, a display section which displays a current counting value, an upper limit setting value display section, and a lower limit setting value display section. The case displays a setting value, an upper limit setting value, and a lower limit setting value. The counter device has the upper limit setting value display section and the lower limit setting value display section which respectively set the upper limit setting value and the lower limit setting value. Even in a case where the setting value is erroneously changed during an operation of the instrument, the operation of a system is smoothly maintained by performing counting-up while using the upper limit setting value or the lower limit setting value which is set to a predetermined setting value. During the operation, the setting value can be changed within a range between the upper limit setting value and the lower limit setting value.